Fly Management HELP!!!!!

I'm not sure if this is the right place to start this thread, but I figured it fell under maintenance, although it's not really regarding the coop because my chickens free range and the problem is the entire yard, for the most part.
I realize that flies are an issue on farms, we've had our little hobby farm now for 13 years and have had chickens, geese, turkeys and ducks free ranging for about the past 10. Well, last year and this year the fly population is out of control. I am literally killing like 15 to 20 flies a day INSIDE the house that are getting in.

The other day as I was out feeding I realized that the problem is the ground outside, there were so many flies around the ground where the poo is. Which makes sense. So I'm thinking if I can control the fly issue outside, to some extent, it will help with the issue inside.

As I've said, we've had free ranging poultry for about 10 years no so the manure area is pretty much most of the ground. Of course with rain and weather, it's not like it's all just a huge pile of poo, but there are still flies. Obviously things like sawdust and DE arent really going to be helpful because I have such a large area to cover. I've read online about products that the chickens can eat when they poop it out it will prevent the larvae that is laid in the poo from developing. Basically within a couple days the fly issue is under control, but I am not having any luck finding any of those products. I have found some very scientific articles on line mentioning some chemical products, but I also have two dogs that are outside sometimes so I want to make sure the prooducts are safe.

Does anyone have any advice as to how I might be able to start to control this issue? Do you think spreading DE on the ground would help? If so I could use one of those spreaders you use for fertilizer or grass seed and spread it across the ground?

I am going nuts with all these flies and any help would be greatly appreciated.

I've used the fly predators for several years, and they do help. We've also got horses and cattle, and I'm not a big fan of spraying everywhere! The chickens will eat the pupa and little wasps in the package, so I put it up out of reach, so they have a chance, and they do travel, so setting them upwind helps too. Mary